Nowadays, popular applications on smartphones have much more sophisticated traffic pattern than what the network architect originally had in mind. It turns out that the existing networks do not support popular applications that well. For example, chattiness of applications where traffic is based on user interaction results in inconstant QoS requirement over time. Another example is “keep alive messages” or background traffic of application or OS where traffic has short and infrequent data sessions. When bringing this type of traffic, it creates a number of issues. First, many always-on applications generate frequent traffic, e.g., for keep alive and status update, which brings significant problems. Second, the signaling in the network has increased a lot due to frequent context establishment and release, e.g., for Connected—Idle transitions. In many networks, this becomes a severe dimensioning problem for the whole network. Third, the UE batter life is short. Finally, for devices that generate always-on sparse traffic, the overhead is very large as compared to the data payload transmission.
In 3GPP LTE/LTE-A systems, operations could be divided to two radio resource control (RRC) states: RRC C_CONNECTED and RRC_IDLE. In RRC_CONNECTED mode, an eNB would keep UE's context (security, id) and process radio resource management (RRM) for that UE. RRM here includes data scheduling, link monitoring (MCS adaption), handover, etc. A UE is ensured to make seamless data transmission with eNB when the UE is in RRC_CONNECTED mode. The eNB may command UE to perform RRM measurement and make handover (HO) decisions after receiving reports that indicate serving cell's signal quality is not good. HO decision is done in an inter-eNB negotiation fashion, including UE context forwarding and initial configuration handled in RRC_CONNECTED mode.
Since radio resources and network capacity are limited (e.g., by backhaul capacity), it is impossible to keep all UEs in RRC_CONNECTED mode. Inactive UEs are therefore released to RRC_IDLE mode. The release decision may depend on (1) Inactivity duration, (2) UE's access priority, and (3) QoS. Once a UE goes to RRC_IDLE mode, eNB also clears its context.
An eNB may configure discontinuous reception (DRX) for a connected UE. Therefore, the UE only needs to monitor the PDCCH at active period. The UE is allowed to sleep (e.g. turn off its TX/RX transceiver) to achieve power saving at sleep period. The DRX operation is controlled by several parameters signaled through RRC message. For uplink transmission, upon packet arrival, UE transmits Scheduling Request (SR) (D-SR on PUCCH or RACH) to inform eNB. In legacy systems, SR transmission is independent of DRX procedure. Thus, SR transmission during DRX degrades the potential power saving since UE may transmit SR at sleep period. Furthermore, when traffic is delay tolerant and small, it is very inefficient to trigger SR during sleep period, since the data can be delayed and there is no reason to decrease the opportunity of power saving or shorten UE battery life.
It has been shown that although there is only 30% data traffic oriented from smartphone devices, they would contribute 80% signaling traffic. This demonstrates that new data applications running on the smartphone have significant impacts on network capacity. Furthermore, it has been observed that the problem comes from so called “background traffic”—due to its small packet and long packet inter-arrival time, current Discontinuous Reception Mechanism (DRX) may not work efficiently. In addition, background traffic also increases the number of RRC state transition, which creates more signaling. As a result, UE battery life is degraded while those applications are running on smartphone devices.
To solve the problem, two issues shall be addressed. First, how to decide when to send the UE to RRC_IDLE mode. In general, the decision relies on a proper RRC release timer that is controlled by eNB. The timer is re-started upon transmitting each packet, and if the timer is expired, UE is released. Usually, a simple default value is used for the timer. Improvement is expected if the timer is adjusted according to on-going traffic. Second, how to decide when to transmit SR. In legacy system, when a packet arrives at layer-2 buffer, UE would transmit the SR as soon as possible. Considering not all packets from smartphones are delay sensitive, SR transmission for delay-tolerant packets can be postponed. Mechanisms for uplink shaping and SR prohibition is thus desirable for additional power saving.